In the current state of the art, variable volume load sensing hydraulic systems are being used on farm and construction equipment as well as various other mobile hydraulic applications. These applications have varying hydraulic demands including the basic power requirements for moving the vehicle. Variable volume load sensing systems are highly efficient and provide substantial horsepower savings over the fixed displacement systems. All systems of this type which operate near the full power capacity of the machine, require some means of horsepower limiting so as not to exceed the capacity of the prime mover and stall the engine.
One alternative to a variable volume system is to use two fixed displacement pumps in parallel with an unloading valve on one pump which dumps the flow to drain from that pump when a pressure level is exceeded. This increases the corner horsepower of the system. One disadvantage of this system is the abrupt flow change when the one pump is unloaded. Another undesirable feature is that the maximum horsepower can only be achieved at two points rather than the infinite choice available with a variable volume system.
The principle disadvantage of variable volume systems is the high cost of the variable volume pump which becomes an even greater factor in the large displacement systems. While there are numerous small variable volume pumps on the market below 3.5 CIR; there are very few larger displacement pumps available and the costs are accordingly very high.